JPH09208219A - Zeolite having mordenite structure and method for producing the same - Google Patents

Abstract

(57) Abstract: A zeolite having a high-performance mordenite structure that can be used as a catalyst is provided. A zeolite having a high-performance mordenite structure can be provided by a production method in which a polyalkylene glycol or a derivative thereof is added to an aqueous reaction mixture at the time of zeolite synthesis to crystallize.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a zeolite having a mordenite structure, which is widely used as a catalyst or an adsorbent for the petrochemical industry and for producing fine chemical products, and a method for producing the same.

[0002]

_2. Description of the Related Art Zeolite has a basic skeleton structure having a three-dimensional network structure in which tetrahedrons of SiO ₂ and AlO ₂ are bonded by sharing oxygen. Therefore, the ratio of oxygen atoms to the total of aluminum atoms and silica atoms, that is, O / (A
1 + Si) is equal to 2. Therefore, the negative electron valence of the tetrahedron containing aluminum maintains electrical neutrality by containing cations such as alkali metal ions, alkaline earth metal ions, ammonium ions, and hydrogen ions in the crystal. Cations can be replaced with other cations by a suitable ion exchange method, which is well known as one of the most important properties of zeolites.

It is also well known that zeolite has a pore structure of molecular order. The pore cavities are generally occupied by water of hydration, and after at least partial dehydration under appropriate conditions, other molecules can be adsorbed and retained in the pore cavities. That is, zeolite has a property as an adsorbent. The size and shape of molecules that can be adsorbed are restricted by the pores. Therefore, it is possible to perform so-called molecular sieving adsorption separation in which a specific molecule is separated from the mixture based on the size or shape of the molecule. In addition to the size and shape of the molecule, there are factors that enable selective adsorption of certain molecules. These factors include, for example, the polarizability of molecules to be adsorbed, the degree of unsaturation, the polarizability within the zeolite pores, the size of cations, and the degree of hydration. These factors enable selective adsorption. You can also

Another property that characterizes zeolites is their significantly higher catalytic activity. In particular, by replacing the ion-exchangeable alkali metal ions of zeolite with ammonium ions, hydrogen ions, or polyvalent metal cations such as rare earth ions, solid acidity is developed, which is a significantly high catalytic action for many reactions. Indicates.

Zeolites generally include natural products and synthetic products. Examples of the natural ones include borofluorite, soda fluorite, kifu fluorite, clinoptilolite, kaijuji fluorite, morden fluorite, ryofus fluorite, and forgesite. As synthetic zeolite, A, B, D, E, F, G, H, J, L, M, Q,
R, S, T, U, X, Y, Z and other types of zeolites can be mentioned.

Most natural products include amorphous or other foreign zeolites or feldspars, crystals such as quartz that are not zeolites, and generally have low crystallinity. Further, these impurities usually block the pores of the zeolite, and the characteristics of the zeolite itself cannot normally function sufficiently.

However, in the case of the synthetic product, the purity of the zeolite can be made extremely high and the pore size is uniform, so that it has superior characteristics as an adsorbent or a catalyst as compared with the natural product. . From such a thing,
As described in detail, many zeolites have been synthesized. Some of them are industrially implemented and used in many applications.

Zeolite synthesis generally involves preparing an aqueous reaction mixture containing a silica source, an alumina source, and an alkali source,
This can be achieved by crystallizing under the reaction conditions that give zeolite.

Generally, the mordenite structure has SiO ₂ and A
lO ₂ is a zeolite having a two-dimensional pore structure consisting of covalently to bonded oxygen 12-membered ring and 8-membered ring of oxygen,
The structure is described in Meyer; Zeitschrift Heuer Crystallography, 115, p439-450 (1961).

The outer surface area of the zeolite having a mordenite structure synthesized by a general method is as follows: Utsunomiya Daigaku Kouyuk Gakubukiyo, Dive 2, 39, p33-
47 (1989) discloses a maximum of 12 m ² / g.

[0011]

Zeolites having a mordenite structure are widely used as catalysts or adsorbents for the petrochemical industry and the production of fine chemical products by taking advantage of their high solid acidity and characteristic pore structure. . However, from the viewpoint of environmental protection and energy saving, it is desired to further reduce the amount of waste generated in the catalytic reaction step by further improving the performance of the zeolite or to efficiently and selectively advance the catalytic reaction.
For example, in the isomerization reaction of an aromatic halide disclosed in Japanese Examined Patent Publication No. 4-37055, neither the reaction activity nor the selectivity has reached a satisfactory level.

[0012]

Means for Solving the Problems As a result of intensive studies for solving the above problems, the present inventors have found that (1) silica source, (2) alumina source, (3) alkali source, (4) fourth
It has been found that a zeolite having a mordenite structure synthesized by adding (5) polyalkylene glycol or a derivative thereof to an aqueous reaction mixture composed of a primary ammonium salt and / or a dicarboxylic acid has superior properties to those of the past. The present invention has been reached. For example, it was found to have high activity for the above-mentioned isomerization reaction. That is, the present invention provides (1) a silica source,
Mordenite structure characterized by adding (5) polyalkylene glycol or its derivative to an aqueous reaction mixture consisting of (2) alumina source, (3) alkali source, (4) quaternary ammonium salt and / or dicarboxylic acid And a method for producing a zeolite having

The zeolite of the present invention comprises (1) a silica source,
It is important to add (5) polyalkylene glycol or its derivative to the aqueous reaction mixture consisting of (2) alumina source, (3) alkali source, (4) quaternary ammonium salt and / or dicarboxylic acid. The method for producing the zeolite of the present invention will be described below.

In the synthesis of the zeolite of the present invention, the silica source is not particularly limited, but silica sol, silica gel, silica gel, silica hydrogel, silicic acid, which are silica sources conventionally used for the synthesis of zeolite and the like,
A silicate ester, a sodium silicate, etc. are used.

The alumina source is not particularly limited, but it is also a well-known alumina source that has been conventionally used in zeolite synthesis, such as sodium aluminate, aluminum sulfate, aluminum nitrate, alumina sol, alumina gel, activated alumina, and gamma-alumina. , Alpha-alumina, etc. are used.

Examples of the alkali source include sodium hydroxide and potassium hydroxide as the inorganic base, and tetraethylammonium hydroxide as the organic base, with sodium hydroxide being preferred. These alkali sources, OH in the system ^- is added to be present in the preferred composition. If necessary excess OH ^-, and can be adjusted sulfate, an organic acid such as a mineral acid or acetic acid, such as hydrochloric acid.

To facilitate the formation of the mordenite structure, a quaternary ammonium salt and / or dicarboxylic acid is added as a template or mineralizer. 4th
Neither the quaternary ammonium salt nor the dicarboxylic acid is particularly limited, but specifically, the quaternary ammonium salt is tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, tetrahydroxyammonium hydroxide. Butyl ammonium and the like can be mentioned, with preference given to tetraethyl ammonium hydroxide. Examples of the dicarboxylic acid include oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, dimethylmalonic acid, methylsuccinic acid, 2,2-dimethylsuccinic acid, maleic acid,
Fumaric acid, glutaconic acid, allyl malonic acid, tartronic acid, malic acid, tartaric acid, pinaconic acid, 1,2-cyclopropanedicarboxylic acid, 1,1-cyclobutanedicarboxylic acid, 1,2-cyclopentanedicarboxylic acid and the like, Tartaric acid is preferred.

In order to synthesize the zeolite having a mordenite structure of the present invention, the above silica source, alumina source,
It is important that the polyalkylene glycol or its derivative be present together with the alkali source, the template agent or the mineralizer. Examples of the polyalkylene glycol include polyethylene glycol and polypropylene glycol. As a derivative of polyalkylene glycol, polyethylene glycol methyl ether,
Polyethylene glycol butyl ether, polyethylene glycol divinyl ether, polyethylene glycol monolauryl ether, polyethylene glycol monooleyl ether, polyethylene glycol monocetyl ether, polyethylene glycol monododecyl ether, polyethylene glycol mono-4-nonylphenyl ether, polyethylene glycol monolaurate, polyethylene Examples thereof include glycol monostearate, polyethylene glycol distearate, polyethylene glycol methacrylate, polyethylene glycol dimethacrylate, polyethylene glycol ethyl ether methacrylate and the like. The average molecular weight of the polyalkylene glycol or its derivative is 500 to 20,000.
0, preferably 1,000 to 10,000
Range. When the average molecular weight is lower than 500, the external surface area becomes 30 m ² / g or less, and it becomes difficult to synthesize the zeolite of the present invention. On the other hand, if it exceeds 20,000, the aqueous reaction mixture will separate into two layers.

The aqueous reaction mixture for producing the zeolite having a mordenite structure of the present invention comprises (1) a silica source, (2) an alumina source, (3) an alkali source, (4) a templating agent or a mineralizer. The quaternary ammonium salt, (5) polyalkylene glycol or its derivative is prepared so as to have the following composition range in terms of molar ratio. _{SiO 2 / Al 2 O 3 =} 5~50 OH / SiO 2 = 0.2~0.45 R / (R + Na) = 0.2~0.5 H 2 O / SiO 2 = 10~50 Q / Al 2 O ₃ = 0.05 to 1.0 (wherein, SiO ₂ is SiO ₂ content of Si in the silica source
Converted moles, Al ₂ O ₃ is the moles of Al in the alumina source converted to Al ₂ O ₃ , OH is an alkali source, R is a quaternary ammonium salt, and Q is a polyalkylene glycol or its derivative. More preferable range is SiO ₂ / Al ₂ O ₃ = 10 to 30 OH / SiO ₂ = 0.2 to 0.3 R / (R + Na) = 0.2 to 0.4 H ₂ O / a _{SiO 2 = 20~40 Q / Al 2} O 3 = 0.05~0.5. When a dicarboxylic acid is used as a template agent or a mineralizer, the aqueous reaction mixture is (1) silica source, (2) alumina source, (3) alkali source, (4)
Dicarboxylic acid as template or mineralizer,
(5) The polyalkylene glycol or its derivative is prepared in the following compositional range in terms of molar ratio.

SiO ₂ / Al ₂ O ₃ = 5 to 50 OH / SiO ₂ = 0.2 to 0.45 A / Al ₂ O ₃ = 0.5 to 4.0 H ₂ O / SiO ₂ = 10 to 50 Q / Al ₂ O ₃ = 0.05 to 1.0 (where SiO _{2 is} the content of Si in the silica source is SiO ₂
Converted number of moles, Al ₂ O ₃ is the number of moles obtained by converting the content of Al in the alumina source into Al ₂ O ₃ , OH is an alkali source, A is a dicarboxylic acid, and Q is each number of polyalkylene glycol or its derivative. It is shown) and a more preferred _{range, SiO 2 / Al 2 O 3} = 10~30 OH / SiO 2 = 0.2~0.3 A / Al 2 O 3 = 1.0~3.0 H 2 O / SiO 2 = 10 to 30 Q / Al ₂ O ₃ = 0.05 to 0.5.

The aqueous reaction mixture thus prepared is heated in the range of 100 to 200 ° C., preferably 140 to 18
Crystallize by reacting in the range of 0 ° C. Reaction temperature 100
When the temperature is lower than 0 ° C, zeolite having a crystal structure other than the mordenite structure may be mixed, or crystallization may be insufficient and the crystal may be amorphous. If the temperature is 200 ° C. or higher, silica crystals may be mixed. Crystallization time is usually 5
It is from time to 30 days, preferably from 10 hours to 10 days. Since the optimum crystallization temperature and crystallization time depend on the composition of the aqueous reaction mixture used, these are appropriately selected.

The reaction mixture is preferably crystallized by placing it in an airtight container, for example, an iron, stainless steel, or Teflon-lined autoclave. Usually, the reaction is carried out under pressure which occurs naturally depending on the temperature. It is desirable that the reaction mixture is always kept homogeneous during crystallization by continuous stirring. Although not particularly limited, the stirring blades are paddle type, arrow paddle type, flat plate type,
Inclined paddle type, propeller type, turbine type, and curved blade turbine type are used, and the stirring speed is 100 to 10
00 rpm, preferably 200 to 600 rpm.
When the rotation speed is 100 rpm or less, sufficient stirring is not performed,
Impurities may be mixed in, and the load on the apparatus increases at 1000 rpm or higher.

After the reaction product thus crystallized is cooled, it is taken out of the closed container and separated from the reaction mother liquor. Then, the zeolite having a mordenite structure of the present invention can be obtained by sufficiently washing with ion-exchanged water and drying at 50 to 250 ° C., preferably 100 to 200 ° C. for 0.5 to 48 hours.

The outer crystal surface area of the zeolite having a mordenite structure according to the present invention is preferably 30 m ² / g or more. The crystal outer surface area of zeolite can be determined by a conventional nitrogen adsorption method after being converted into a proton-type zeolite by an ion exchange method or the like in advance.
Specifically, the zeolite is pretreated at 350 ° C. for 1 hour under vacuum, and then nitrogen is introduced at a liquid nitrogen temperature to measure a Langmuir type nitrogen adsorption isotherm. This nitrogen adsorption isotherm is described by Seiichi Kondo, Tatsuo Ishikawa, and Ikuo Abe, “Chemistry Seminar 16: Chemistry of Adsorption”, p52.
By analyzing using the t method disclosed in (1991), the crystal surface area of the zeolite can be obtained. The zeolite of the present invention has an outer crystal surface area of 30 m ^2.
/ G or more and 50 m ² / g or less.

Table 1 shows the X-ray diffraction pattern of the zeolite having a mordenite structure of the present invention.

[0026]

[Table 1]

The zeolite having a mordenite structure of the present invention can be used for various organic compound conversion reactions. For example, reforming, paraffin isomerization, aromatic compound isomerization, hydrogenation / dehydrogenation, cracking, toluene disproportionation and the like can be mentioned. It is particularly useful for the isomerization reaction of aromatic compounds. Examples of aromatic compounds include aromatic alkyl compounds and aromatic halides. Examples of the aromatic alkyl compound include xylene, diethylbenzene, and methylethylbenzene. Examples of aromatic halides include dichlorobenzene, trichlorobenzene, dichlorotoluene, trichlorotoluene, dichloroethylbenzene, trichloroethylbenzene, dibromobenzene, dibromotoluene, dibromoethylbenzene, bromochlorobenzene, bromochlorotoluene and the like.

When the zeolite having a mordenite structure of the present invention is used for the isomerization reaction of an aromatic compound, it is usually molded and the cations in the zeolite are ion-exchanged with hydrogen ions. Reaction temperature is usually 150-5
Although it is about 00 ° C, 200 to 450 ° C is particularly preferable. The reaction pressure is not particularly limited, but it goes without saying that in the case of liquid phase reaction, the reaction pressure must be set in order to keep the aromatic compound in the liquid phase state. Weight hourly space velocity (WHSV) is 0.05 to 1 based on aromatic compounds
It is 0 hr ^-1 , preferably 0.1 to 5 hr ^-1 .

[0029]

The present invention will be described below with reference to examples, but the present invention is not limited to these.

(Zeolite synthesis) Example 1 252 g of ion-exchanged water and 11.0 g of polyethylene glycol (Tokyo Kasei, molecular weight 6000) were placed in a resin beaker and well dissolved. Then, after adding 2.5 g of sodium hydroxide (Katayama Chemical Co., Ltd.) and dissolving it well, tetraethylammonium hydroxide (Sanyo Kasei EAH-20; content = 1)
9.9 wt%, the H ₂ O = 80.1wt%) 36.1g
added. Next, sodium aluminate (Sumitomo Chemical; N
aOH = 26.1 wt%, Al ₂ O ₃ = 18.5 wt%,
After adding 14.3 g of H ₂ O = 55.4 wt%) and stirring well, finally, hydrous silicic acid powder (Nippon Silica Industry; Si
O ₂ = 93.3%, H ₂ O = 6.7 wt%) 41.9 g was added. The composition of this slurry-like mixture was as follows in terms of molar ratio.

SiO ₂ / Al ₂ O ₃ = 25 OH / SiO ₂ = 0.3 R / (R + Na) = 0.245 H ₂ O / SiO ₂ = 25 Q / Al ₂ O ₃ = 0.075 (however, R represents a tetraethylammonium salt and Q represents polyethylene glycol.) This slurry was put in a stainless steel autoclave having an internal capacity of 500 cc, and reacted at 170 ° C. for 71 hours with sufficient stirring. The produced zeolite was thoroughly washed with distilled water and then dried at 120 ° C. overnight.

As a result of measuring the obtained white powder by an X-ray diffraction method, it was found to be substantially the same as the X-ray diffraction pattern of the mordenite type zeolite shown in Table 1, and it was confirmed that zeolite having another crystal structure was mixed. I couldn't do it.

Example 2 A zeolite was synthesized in the same manner as in Example 1 except that polyethylene glycol monostearate (Tokyo Kasei, molecular weight 2700) was used instead of polyethylene glycol. However, the ion-exchanged water used was 253 g, polyethylene glycol monostearate 5.3 g, sodium hydroxide 2.6 g, tetraethylammonium hydroxide aqueous solution 36.1 g, sodium aluminate 14.4 g, and hydrous silicic acid powder 42.4 g. did. The composition of this slurry-like mixture was as follows in terms of molar ratio.

SiO ₂ / Al ₂ O ₃ = 25 OH / SiO ₂ = 0.3 R / (R + Na) = 0.245 H ₂ O / SiO ₂ = 25 Q / Al ₂ O ₃ = 0.075 (however, R represents a tetraethylammonium salt and Q represents polyethylene glycol monostearate.) The white powder obtained was measured by X-ray diffractometry. As a result, the X-ray diffraction pattern of the mordenite-type zeolite shown in Table 1 was substantially the same. Therefore, the inclusion of zeolite having another crystal structure was not recognized.

Comparative Example 1 279 g of ion-exchanged water was put in a beaker made of resin, and then 5.9 g of sodium hydroxide (Katayama Chemical Co., Ltd.) was added and well dissolved, and then tetraethylammonium hydroxide (SANYO Kasei EAH-20; content = 19). .9 wt%, H ₂ O = 80.
1 wt%) 31.2 g was added. Next, sodium aluminate (Sumitomo Chemical; NaOH = 26.1 wt%, Al ₂
O ₃ = 18.5 wt%, H ₂ O = 55.4 wt%) 15.
After adding 5 g and stirring well, hydrous silicic acid powder (Nippon Silica Industry Co., Ltd .; SiO ₂ = 93.3%, H 2 O = 6.
7 wt%) 45.1 g was added. The composition of this slurry-like mixture was as follows in terms of molar ratio.

SiO ₂ / Al ₂ O ₃ = 25 OH / SiO ₂ = 0.4 R / (R + Na) = 0.15 H ₂ O / SiO ₂ = 25 (where R represents a tetraethylammonium salt) The slurry was put into a stainless steel autoclave having an internal capacity of 500 cc, and the reaction was carried out at 170 ° C. for 74 hours with sufficient stirring. The produced zeolite was thoroughly washed with distilled water and then dried at 120 ° C. overnight.

The white powder obtained was measured by X-ray diffractometry. As a result, the X-ray diffraction pattern of the mordenite type zeolite shown in Table 1 was substantially the same, and the inclusion of zeolite having another crystal structure was confirmed. I couldn't do it.

Comparative Example 2 Synthesis was carried out in the same manner as Comparative Example 1. However, the ion-exchanged water used was 283 g, sodium hydroxide 7.4 g, tetraethylammonium hydroxide aqueous solution 88.7 g, sodium aluminate 30.8 g, and hydrous silicic acid powder 90.4 g. The composition of this slurry-like mixture was as follows in terms of molar ratio.

SiO ₂ / Al ₂ O ₃ = 25 OH / SiO ₂ = 0.35 R / (R + Na) = 0.245 H ₂ O / SiO ₂ = 15 (wherein R represents a tetraethylammonium salt) The slurry was put into a stainless steel autoclave having an internal capacity of 500 cc, and the reaction was carried out at 170 ° C. for 68 hours with sufficient stirring. The produced zeolite was thoroughly washed with distilled water and then dried at 120 ° C. overnight.

The white powder obtained was measured by an X-ray diffraction method. As a result, the X-ray diffraction pattern of the mordenite type zeolite shown in Table 1 was substantially the same, and the inclusion of zeolite having another crystal structure was confirmed. I couldn't do it.

(External Crystal Surface Area) Examples 1 and 2, Comparative Example 1
~ 2 zeolite 5.4g in a glass beaker,
25 ml of 10 wt% ammonium chloride aqueous solution was added.
This was placed in a water bath at 80 ° C., left for 1 hour and then filtered. The mordenite type zeolite synthesized by repeating this operation 5 times was designated as an ammonium type. This is 50
Proton type was obtained by firing at 0 ° C. for 2 hours. The outer crystal surface area of the zeolite was measured after the zeolite was thus converted into the proton form. The measurement was performed using a high-precision fully automatic gas adsorption device BELSORP36 manufactured by Nippon Bell. Table 2 shows the results.

[0042]

[Table 2] As is clear from Table 2, it was found that a zeolite having a mordenite structure having an outer crystal surface area of 30 m ² / g or more can be synthesized by adding polyalkylene glycol or its derivative during the synthesis of zeolite.

(Catalyst Preparation) Catalysts 1 to 3 The zeolites having a mordenite structure obtained in Examples 1 and 2 and Comparative Example 1 were designated as Catalysts 1 and 2 and Catalyst 3 as follows. Alumina sol in 100 parts by weight of zeolite powder (Nissan Chemical No. 200; Al ₂ O ₃ = 10 wt%)
After adding 150 parts by weight and molding, the mixture was dried at 120 ° C. overnight and then baked at 500 ° C. for 2 hours. This mordenite molded body was heated at 80 ° C. using a 10 wt% ammonium chloride solution.
The ion exchange was performed 5 times. The ammonium ion-exchanged zeolite was dried at 120 ° C. overnight and then 1N-NH
_It was treated with a ₄ F aqueous solution at a solid-liquid ratio of 3.0 liter / kg at room temperature for 3 hours. After draining, 1.88 parts by weight of an aqueous rhenium peroxide solution (rare metal; containing Re ₂ O ₇ = 40 wt%) was collected and treated at a solid-liquid ratio of 2 liter / kg and room temperature for 3 hours. After draining, the solution was dried at 120 ° C. overnight and then baked at 540 ° C. for 3 hours.

(Evaluation of catalyst performance) Examples 3 to 4 Using catalysts 1 to 2 in a fixed bed flow reactor, a dichlorotoluene (DCT) mixture was subjected to isomerization reaction.
The reaction conditions and the reaction results are shown in Table 3.

Comparative Example 3 An isomerization reaction of a dichlorotoluene (DCT) mixture was carried out using a catalyst 3 using a fixed bed flow reactor. The reaction conditions and the reaction results are shown in Table 3.

[0046]

[Table 3] As is clear from Table 3, 2,5-DCT, 2,4-D
By contacting a raw material having a high CT content with the zeolite of the present invention, 2,5-DCT, 2,4-DCT are 3,5-DCT, 2,6-DCT, 3,4-DCT,
It was found that isomerization was efficiently performed to 2,3-DCT.

[0047]

INDUSTRIAL APPLICABILITY According to the present invention, a zeolite having a mordenite structure crystallized by adding polyalkylene glycol or a derivative thereof to an aqueous reaction mixture at the time of synthesizing zeolite is a catalyst for various organic compound conversion reactions, For example, it can be effectively used as a catalyst for isomerization reaction of aromatic halides.

Claims (6)

[Claims] 1. A (1) silica source, (2) an alumina source,
(3) In an aqueous reaction mixture comprising an alkali source, (4) quaternary ammonium salt and / or dicarboxylic acid (5)
A method for producing a zeolite having a mordenite structure, which comprises adding a polyalkylene glycol or a derivative thereof. 2. The addition amount of polyalkylene glycol or its derivative is expressed by a molar ratio Q / Al ₂ O ₃ = 0.05 to 1.0 (where Al ₂ O ₃ is contained in the alumina source). _2. The production method according to claim 1, wherein the content of Al is the number of moles converted to Al ₂ O ₃ , and Q is the number of moles of polyalkylene glycol or its derivative. 3. The production method according to claim 1, wherein the polyalkylene glycol or its derivative is selected from polyethylene glycol, polypropylene glycol or their derivatives. 4. The method according to claim 1, wherein the polyalkylene glycol or its derivative has an average molecular weight in the range of 500 to 20,000. 5. The aqueous reaction mixture is heated to 100 ° C. to 200 ° C.
Crystallization is performed at the temperature of 5. The manufacturing method according to claim 1. 6. A zeolite having a mordenite structure crystallized by the method according to claim 1.
Zeolite having an outer crystal surface area of 30 m ² / g or more.